DCHP-1 Online
cayuse ((n.)) [< the Cayuse Indians of parts of Washington and Oregon, U.S.A.] 
n. Originally, a wild pony, a mustang; later generalized in the West as a name for any horse, often with a derogatory or contemptuous connotation.
See also: Indian horse Indian pony shaganappi pony squaw horse
- 1860 A few uninitiated Oregonians . . . had rode over on Keyuse poneys from the Willamette valley.
- 1863 We were amused by observing a drove of "Kioush" horses that had been driven from the mountains in a wild state, and were enclosed in a yard. . . .
- 1962 "Well, chances are Mr. Dude, you've been getting a silent horse laugh yourself from your own trusty cayuse."
- 1860 A few uninitiated Oregonians . . . had rode over on Keyuse poneys from the Willamette valley.